I think my DS is quite small. He is 16kg & 100cm. He turned 4 this month. He's a skinny little thing (still wears size 1 shorts ) but he rarely gets sick. He's healthy & active. My family are all on the small side so I think it is in his genes.

Is your family or your DH's family small? If so, I would be more concerned about your DS being ill a lot than his size. I think if you are worried, then you should get him checked out.